Intercluster porous nanocomposites from combination of [AlO_4Al_(12)(OH)_(24)(H_2O)_(12)]~(7+) cation and Anderson-type [Al_(1-x)Cr_xMo_6O_(24)H_6]~(3-) anion: XRD, TG-DTA, SEM-EDAX and EPR studies

摘要

The synthesis of intercluster porous nanocomposites obtained from polyoxometallate compounds such as the [AlO_4Al_(12)(OH)_(24)(H_2O)_(12)]~(7+) cation (named Al_(13)) and the Anderson-type [Al_(1-x)Cr_xMo_6O_(24)H_6]~(3-) anion (named Al_(1-x)Cr_xMo_6) has been performed in order to study the interaction between the two cluster ions, the stability of the XMo_6 planar configuration, the products obtained after thermal treatment, the structure and the local symmetry of the Cr~(3+) species. Chemical, thermal, structural and spec-troscopic characterizations of the original and thermally treated phases have been followed by different techniques such as TG-DTA, XRD, SEM-EDAX, and mainly by EPR. All the results have shown that the structure of the intercluster nanocomposites (Al_(13))(Al_(1-x)Cr_xMo_6)_2 precursors is similar to that reported by Son et al. for the chromium-free (Al_(13))(AlMo_6)_2 intercluster nanocomposite [Son et al., J. Am. Chem. Soc. 122 (2000) 7432]. After thermal treatment in air at several temperatures of the (Al_(13))(Al_(1-x)Cr_xMo_6)_2 nanocomposites the following phases have been observed and characterised: (ⅰ) at 400℃ an amorphous phase containing dispersed Cr~(3+) ions; (ⅱ) at 700℃ a crystalline phase corresponding to Cr_2(MoO_4)_3/Al_2(MoO_4)_3 solid solutions; (ⅲ) at 950℃ α-Al_2O_3/Cr_2O_3 solid solutions with a random dispersion of the Cr~(3+) ions.